1. Field of the Invention
The present invention relates to a waveform equalizing circuit for shaping the waveform of a signal read out by the head of a recorder or the like to enhance the reliability of the readout data.
2. Description of the Prior Art
When information is recorded in a magnetic disk device at recording densities above the appropriate level, interference of the readout signals can take place, lowering the voltage level or shifting the pattern peak, resulting in lowered accuracy or reliability of the signal detection. For this reason, a waveform equalizing circuit has been used in the readout circuit to decrease the half-value width, thereby reducing the waveform interference and the degree of the above voltage drop or pattern peak shift.
FIG. 3 shows an example of the prior-art waveform equalizing circuit described in the National Convention Record 1979, the Institute of Electronics and Communication Engineers of Japan, 201. A delay line 2 having a delay time .tau. and a characteristic impedance Ro is connected to the positive terminal of a differential amplifier 1 having a very large input impedance. A signal source 3 equivalent to the readout signal of a magnetic head is connected to the delay line 2 through a resistance circuit 4 consisting of the first, second, and third resistors 4a, 4b, and 4c, having resistance Ra, Rb, and Rc, respectively. These resistors not only determine the attenuation factor K but also serve as a matching circuit with the characteristic impedance Ro of the delay line 2.
In operation, suppose a solitary wave E(t) is fed to the waveform equalizing circuit from the signal source 3. E(t) is generally expressed in terms of the differential arc tangent as follows: EQU E(t)=1/{1+(t/W.sub.50).sup.2 } (1)
where W is the half-value width or the time width of the solitary wave at which its voltage level is 50% of the peak value.
When this signal E(t) is fed to the delay line 2, the voltage Ea(t) at the matching terminal a is given by EQU Ea(t)={E(t)+E(t-2.tau.)}.times.(Rb+Rc)/2(Ra+Rb+Rc) (2)
This voltage Ea(t) is divided by the resistors 4b and 4c to provide a voltage Eb given by EQU Eb(t)={E(t)+E(t-2.tau.)}.times.Rc(Rb+Rc)/2(Rb+Rc)(Ra+Rb+Rc) (3)
which is fed to the negative terminal b of the differential amplifier 1. The reflection end c of the delay line 2 is not matched with the characteristic impedance Ro or forms an open end, and so a signal fed at the matched terminal a is totally reflected at the reflection end c with the delay time .tau.. The magnitude of the voltage produced at the reflection end c is twice that of the signal voltage.
When E(t) is fed from the signal source 3, the voltage Ec(t) applied to the positive terminal c of the differential amplifier 1 is given by EQU Ec(t)=E(t-.tau.)(Rb-Rc)/(Ra+Rb+Rc) (4)
Consequently, the voltage Ed(t) obtained at the output terminal d of the differential amplifier 1 is given by ##EQU1## where Go is the gain of the differential amplifier 1. Letting EQU K=Rc/2(Rb+Rc) (7) EQU G=Go(Rb+Rc)/(Ra+Rb+Rc) (8) EQU t.sub.1 =t-.tau. (9)
then, Eq. (6) is written as EQU Ed(t.sub.1 +.tau.)=G[E(t.sub.1)-K{E(t.sub.1 +.tau.)+E(t.sub.1 -.tau.)}](10)
Eq. (10) represents the output signal from the waveform equalizing circuit, indicating that this waveform equalizing circuit puts out a signal G times as large as the difference between the input signal E(t) and the sum of the delayed signal E(t-.tau.) and the advanced signal E(t+.tau.) multiplied by the attenuation factor K.
This waveform equalizing effect is shown in FIG. 4. L, M, and N represent the input solitary waveform having the half-value width W.sub.50, the signal advanced for the time .tau. and having the magnitude K times as large as the input signal, and the signal delayed with the time .tau. and having the magnitude K times as large as the input signal, respectively. O represents the equalized solitary waveform or the difference between the signal L and the signals M and N, indicating that the half-value width after the equalization is smaller than before the equalization.
The above circuit is useful for symmetrical solitary waveforms such as hown in FIG. 4, which are usually obtained from a ferrite magnetic head or the like, to reduce the half-value width. However, when it is used for an asymmetrical solitary waveform with an undershoot, such as shown in FIG. 5, which is usually obtained from a thin-film head or the like, the equalized waveform has an increased undershoot at its head and/or tail portion because both attenuation factors for the advanced and delayed signals M and N are equal, and these undershoots sometimes have been mistaken as peaks by the peak detector.
Consequently, for a solitary waveform with an undershoot at its tail portion as shown in FIG. 6, a waveform equalizing circuit having a termination resistor 5 with resistance RL at the reflection end c of the delay line 2 as shown in FIG. 7 has been used. When a voltage E(t) is fed from the signal source 3, the voltage Ec(t) generated at the reflection end c is given by EQU Ec(t)=E(t-.tau.){(1+(RL-Ro)/(RL-Ro)}.times.(Rb+Rc)/2(Ra+Rb+Rc) (11)
Since the input signal and reflected signal are combined at the matched end a, the voltage Ea(t) generated at the matched end is given by EQU Ea(t)={E(t)+E(t-2.tau.)(RL-Ro)/(RL+Ro)}.times.(Rb+Rc)/2(Ra+Rb+Rc) (12)
This voltage Ea(t) is divided by the resistors 4b and 4c to provide the negative terminal b of the differential amplifier 1 with a voltage Eb(t) given by EQU Eb(t)={E(t)+E(t-2.tau.)(RL-Ro)/(RL+Ro)}.times.Rc(Rb+Rc)/(Rb+Rc)(Ra+Rb+Rc) (13)
Thus, the voltage Ed(t) generated at the output terminal d of the differential amplifier 1 is given by EQU Ed(t)=Go[E(t-.tau.){1+(RL-Ro)/(RL-Ro)}-E(t)Rc/(Rb+Rc)-E(t-2.tau.).times.Rc( RL-Ro)/(Rb+Rc)(RL-Ro)] (14)
where Go is the gain of the differential amplifier 1. Letting EQU G=Go{1+(RL-Ro)/(RL-Ro)}.times.(Rb+Rc)/2(Ra+Rb+Rc) (15) EQU K.sub.1 =Rc/(Rb+Rc){1+(RL-Ro)/(RL+Ro)} (16) EQU K.sub.2 =Rc(RL-Ro)/(Rb+Rc)(RL+Ro)/{1+(RL-Ro)/(RL+Ro)} (17) EQU t.sub.1 =t-.tau. (18)
then, Eq. (14) is expressed as EQU Ed(t.sub.1 +.tau.)=G{E(t.sub.1)-K.sub.1 E(t.sub.1 +.tau.)-K.sub.2 E(t.sub.1 -.tau.)} (19)
That is to say, the output signal from the waveform equalizing circuit of FIG. 7 is equalt to G times as large as the difference between the original signal E(t) and the sum of the advanced signal multiplied by K.sub.1 and the delayed signal multiplied by K.sub.2 which is smaller than K.sub.1. This waveform equalizing effect is also shown in FIG. 6. L, M, N, and O represent the asymmetric solitary waveform, the signal advanced by .tau. and multiplied by K.sub.1, the signal delayed by .tau. and multiplied by K.sub.2, and the equalized solitary waveform, respectively. Thus, the undershoot of the equalized waveform is controlled by decreasing the value of K.sub.2 while the half-value width is reduced.
FIG. 8 shows a waveform having an undershoot at its head portion. When the waveform equalizing circuit of FIG. 3 or 7 is used for this waveform, the head undershoot is increased so that it can be mistaken as a peak in the peak detection stage. When the head undershoot increase is suppressed, the half-value width cannot be decreased satifactorily. When the half-value width is decreased satisfactorily, this prior art waveform equalizing circuit increases the head undershoot of a readout solitary waveform, thus presenting a new problem of detection of the pseudo peak.